A commercially important carbonylation reaction, using hydrogen as coreactant, is the hydroformylation of olefins, which are reacted with carbon monoxide and hydrogen to form aldehydes and/or alcohols having one carbon atom more than the precursor olefin. Depending on catalyst, reaction conditions and substrates, the hydroformylation can proceed with varying selectivities to the several possible isomeric aldehydes or alcohols in varying yields, as side reactions occur to a smaller or larger extent. Generally only one isomeric product is preferred. For many applications the presence of branched aldehydes or alcohols is undesirable. Moreover, in view of biological degradability, it is considered advantageous to obtain products having a high content of the linear isomer. The selectivity towards one of several possible isomeric products is called regioselectivity. For hydroformylation a regioselectivity towards reaction at the primary carbon atom, resulting in a linear product, is desirable.
WO-A-95/05354 describes the carbonylation of ethylenically unsaturated compounds by reaction with carbon monoxide and hydrogen, i.e. hydroformylation, in the presence of a catalyst system comprising a Group VIII metal cation, viz. cationic palladium and platinum, and a bidentate ligand, viz. a diphosphine. In the examples amongst others 1,2-bis(1,4-cyclooctylene phosphino)ethane, i.e. in IUPAC nomenclature 1,2-PP′bis(9-phosphabicyclo[4.2.1]nonyl)ethane; 1,3-bis(1,4-cyclooctylenephosphino)propane, i.e. in IUPAC nomenclature 1,3-PP′bis(9-phosphabicyclo[4.2.1]nonyl)-propane; and 1,2-bis(2,6-dimethyl, 1,4-cyclooctylenephosphino)ethane, i.e. in IUPAC nomenclature 1,2-PP′bis(2,6-dimethyl, 9-phosphabicyclo[4.2.1]nonyl)-ethane are used as bidentate diphosphine ligands. The phosphabicyclononyl groups in these ligands are all substituted or non-substituted 1,4-cyclooctylenephosphino groups, i.e. in IUPEC nomenclature 9-phosphabicyclo[4.2.1]nonyl groups. Such a 9-phosphabicyclo[4.2.1]nonyl group is visualised in FIG. A.

As is illustrated by the examples the hydroformylation of ethylenically unsaturated compounds with a catalyst system containing these diphosphines results in acceptable selectivities towards the linear product.
The 9-phosphabicyclo[4.2.1]nonyl group visualized in FIG. A is an example of an asymmetrical phosphabicycloalkyl group. In an asymmetrical phosphabicycloalkyl group the bridges not containing the phosphorus atom have an unequal number of atoms in the bridge. By a symmetrical phosphabicycloalkyl group is understood that the bridges (i.e. the hydrocarbyl groups connecting the tertiary carbon atoms), which do not contain the phosphorus atom, have an equal number of atoms. An example of such a symmetrical group is the 9-phospha-bicyclo[3.3.1]nonyl group which is visualised in FIG. B.

WO-A-00/02375 describes a method to prepare a phosphorus-containing ligand by refluxing a phosphabicyclononane hydride with 1,2-dibromoethane in acetonitrile. After neutralisation with sodium hydroxide a bis-(9-phosphabicyclononyl)ethane can be isolated. The phosphabicyclononane hydride can conveniently be prepared as described by Elsner et al. (Chem. Abstr. 1978, vol. 89, 180154x).
In addition, non-pre-published WO-A-01/87899 describes the preparation of a bidentate diphosphine ligand by reacting P-cyclo-octyl hydride (e.g. phosphabicyclononane hydride) and butyllithium to generate a lithium cyclo-octyl phosphide and subsequently reacting with an appropriate substituted or non-substituted alkane diol sulphate ester. The P-cyclo-octyl hydride can conveniently be prepared as described by Elsner et al. (Chem. Abstr. 1978, vol. 89, 180154x).
In their article entitled “A simple procedure for the separation of the catalytically important phobane isomers”, published in Chemical Communications, 1997, pages 1527–1528, J. H. Downing et al. indicate that to that date there had been no reports of the separation of the symmetrical and asymmetrical isomers of phosphabicyclononanes, although, by exploiting the difference in reactivity between the isomers, ligands derived from the symmetrical isomer had been isolated.
In the article of J. H. Downing et al. a laborious method is provided for separation of the isomers of phosphabicyclononane. The method comprises:    a) reacting a mixture of both symmetrical and asymmetrical phosphabicyclononane hydride with formaldehyde (CH2O) in the presence of hydrochloric acid (HCl), yielding phosphonium salts;    b) reacting these phosphonium salts with sodium hydroxide (NaOH), yielding a charged symmetrical phosphine and a neutral asymmetrical phosphine;    c) extracting the neutral asymmetrical phosphine with pentane, leaving relatively pure, charged symmetrical phosphine in an aqueous solution;    d) treating the aqueous solution with sodium hydroxide to obtain the neutral symmetrical phosphine.
The symmetrical phosphabicyclononane is used in the synthesis of 1,3-PP′bis(9-phosphabicyclo[3.3.1]nonyl) propane. The overall yield of this preparation was only 17%. The article does not describe the preparation of any other bidentate diphosphine having general formula I.
Although good results with regard to the regioselectivity towards a linear product are obtained in WO-A-95/05354, there is room for further improvement.
It is therefore desirable to provide a process for the carbonylation of ethylenically unsaturated compounds by reaction with carbon monoxide and a coreactant, which results in an improved regioselectivity towards a linear product.